Early external defibrillators delivered alternating current from a mains circuit, e.g., a building alternating current (ac) power circuit, to a patient. These early defibrillators delivered a long burst, e.g., one or two seconds, of current at the mains voltage, e.g., 120 volts in the United States. Because these early defibrillators relied on the presence of a mains circuit receptacle for power, they were not portable, and could not reach many patients that required defibrillation.
These early defibrillators were eventually replaced by the type of defibrillator that is currently commercially available, i.e., external defibrillators use a direct current (dc) power source, e.g., a battery, as the source of energy for defibrillation of a patient. The dc power sources used by commercially available defibrillators allow them to be self-contained and completely portable. Some commercially available defibrillators may be plugged into a receptacle to receive alternating current from a building circuit, but merely use the current to recharge the dc power source.
Commercially available external defibrillators typically include one or more capacitive elements that store a charge from the dc power source, which is then delivered to the patient in the form of a defibrillation pulse. These defibrillators may also include switches or other wave-shaping circuits in order to deliver the exponentially decaying monophasic pulse output by the capacitive elements as a truncated exponentially decaying monophasic, biphasic, or multiphasic pulse. The defibrillation pulses delivered by commercially available defibrillators have a significantly higher voltage and are significantly shorter than the mains voltage bursts delivered by the early defibrillators and, for that reason, are considered to be more therapeutically effective than the mains voltage bursts delivered by the early defibrillators.
However, because these commercially available external defibrillators include a battery, capacitive elements, and charging circuitry to quickly charge the capacitive elements to a high voltage from the battery, they may be significantly heavy and expensive. Further, defibrillators, such as automated external defibrillators (AEDs), are increasingly being provided at locations, such as homes and small offices, where having a self-contained power source for the sake of portability is not a necessity. Consequently, for such locations, it may be desirable to provide a defibrillator that is not self-contained, but which provides high voltage defibrillation pulses with similar efficacy to those provided by commercially external defibrillators, and may be lighter and less expensive than commercially available defibrillators.
U.S. Published Patent Application No. 2004/0143297 by Ramsey III describes a defibrillator that does not require a battery. Instead, the defibrillator described by Ramsey III charges an array of capacitors directly from a mains circuit, and delivers the energy stored by the capacitors to a patient as a defibrillation in much the same manner as commercially available defibrillators. The defibrillator described by Ramsey III charges the capacitors in parallel, then discharges the capacitors in series to provide a voltage that is significantly higher than that mains voltage for the defibrillation pulses.
Because the defibrillator described by Ramsey III would not include a battery, it might be lighter and less expensive than commercially available defibrillators. However, because the defibrillator described by Ramsey III requires several high value capacitors in order to achieve the high voltage desired for efficacious defibrillation, it would likely be more expensive than a defibrillator that delivered energy directly from a mains circuit to a patient, like the early defibrillators. Consequently, a defibrillator that delivers energy directly from a mains circuit to a patient, like the early defibrillators, but does so in the form of high voltage “pulses” with a desired efficacy, like commercially available defibrillators, might provide the most desirable option in terms of weight and cost for location and applications that do not require mobility.
However, even assuming that such a defibrillator were to become available, its use may be limited by the inability of ac mains circuits in homes, offices and other buildings to deliver adequate energy, e.g., power, voltage, current or charge, for defibrillation. For example, typically a peak current of approximately 10 amps to approximately 20 amps is required to be delivered to a patient for defibrillation. Patient impedances generally fall within the range from approximately 25 ohms to approximately 150 ohms, resulting in a peak power required for defibrillation of a patient that is within a range from approximately 2500 watts to 60,000 watts. Therefore, the peak current draw from a 120 volt ac mains circuit would be within a range from approximately 20 amps to approximately 500 amps.
The resistance of an ac mains circuit in a building may be too high to reliably provide peak current throughout this range. For example, according to the well-known relationship between voltage, current and resistance, a 120 volt ac mains circuit with a resistance of 1 ohm will only be capable of providing an instantaneous current of 120 amps. In some cases, a resistance of as little as 0.1 ohm may render a circuit unable to support delivery of defibrillation pulses to a particular patient, with a particular patient resistance, at a specified energy level.
The resistance of a building circuit may be too high to support defibrillation due to, for example, improper wiring or degradation of materials over time. Further, the fact that a circuit is unable to provide adequate energy for defibrillation, or at least unable do so reliably and for a full range of defibrillation pulse energy levels, will likely not be apparent to the purchaser, installer, or user of a defibrillator that is coupled to that circuit. Nonetheless, because defibrillators are critical, life-saving devices, it is imperative that they be capable of reliably delivering defibrillation pulses at any desired energy level.